A known soil working device comprises a machine frame and a drive for at least one piercing tool that can be lifted and lowered, said piercing tool being operable to be pierced into the soil and thereafter to be withdrawn. A guiding element, e.g. a support arm, for at least one piercing tool is moveably guided on the machine frame. Prior to piercing, the piercing tool is arranged in a starting position under a predetermined piercing angle. A tool holder holds one or a plurality of piercing tools. The tool holder is supported around a first pivot axis in said guiding element which can be lifted and lowered by the drive, so that during engagement with the soil, i.e. under application of force, the piercing tool can be pivoted relative to the guiding element. Upon release of the applied force, a device exerts a restoring moment on the tool holder that acts around the first pivot axis, so that the piercing tool after withdrawal from the soil will pivot back again into the starting position.
Devices of the above type serve for forming cavities in the soil with the aid of piercing tools, wherein slit-shaped cavities will be generated in the depth regions of the soil and holes of the smallest possible size will remain on the surface. Said cavities allow for an improved drainage of water and enhance the aeration of the soil, and the loosening of the soil is effective to improve the growth of plants, especially of grasses.
From European Patent Application No. EP-A-0037595, there is known a soil working device comprising two support arms, guided in the manner of a parallelogram, which on one end pivotably carry a tool holder and on the other end are pivotably supported on the machine frame. The pushing rod of the crank drive is articulated to the tool holder and will drive the same, thus causing it to perform an upward and downward movement. One of the two support arms is adjustable in length and includes a restoring spring with a stopper. As long as the piercing tool is outside the soil, the support arm is in abutment on the stopper due the spring force. In the situation where the piercing tool is in the pierced state and the soil working device is being moved on, the piercing tool together with the tool holder will be pivoted opposite to the travel direction, and the length-adjustable support arm will be extended against the spring force.
In the soil working devices known as of yet, spring elements are used which exert a restoring moment on the piercing tools so that the piercing tools, once they are outside the soil, will move back into their starting position again. Hereunder, the starting position is to be understood as that position which said at least one piercing tool assumes briefly prior to piercing. Outside the soil, however, under the effect of the mass inertia and of the upward and downward movement, the moment of inertia of the tool holder and the piercing tool will act against the restoring moment of the spring. For this reason, use is made of spring elements of high spring stiffness in order to move the piercing tools back into their starting position again. The spring elements with high spring stiffness, however, cause an increased risk that the piercing tools, while still within the soil, will cut slits into the soil surface instead of generating the desired small holes.